In optical transmission systems different kind of protection functions are implemented in order to ensure a minimum of traffic loss. The important parts which are to be protected include for instance the optical transmission fibres.
One type of optically protected transmission system is the 1+1 MSP (Multiplex Section Protected) SDH (or SONET) point-to-point system. Such a protected SDH system comprises two pairs of optical fibres connecting two nodes 1, see FIG. 1. A first pair 3 is the working pair, one fibre of the pair normally carrying light signals in one direction and the other fibre of the pair carrying light signals propagating in the opposite direction. The second pair 5 is the protecting pair, the fibres of this pair normally carrying light signals which are not useful or at least are not used. In a protected SDH terminal multiplexer or node 1 two sets of optical receivers and transmitters are provided, one set 7 for the normally working fibre pair 3 and another set 9 for the protecting fibre pair 5. The transmitter in each set can always be active. The receiver of the set provided for the protecting pair is normally not active. In the case of a break of the fibres in the working pair an electric selector, not shown, decides to activate the receiver of the set for the protecting fibre pair and to deactivate the receiver of the set for the working pair. Thus the traffic is now communicated via the protecting fibre pair and the traffic is restored.
For wavelength division multiplexed (WDM) systems, one implementation of fibre protection is illustrated in the diagram FIG. 2. The system shown is an optical WDM point-to-point system having 1+1 optical multiplex section protection (1+1 OMSP). Only the optical fibres for carrying light signals in one direction between the nodes 11 are shown in FIG. 2, an identical system being used for traffic in the opposite direction.
From each client equipment 13 an optical signal goes to a transmit end transponder (TET) 15. The wavelength specific signal output from the TET 15 then goes to a multiplexer (MUX) 17 in which it is combined with the wavelength specific signals output from other transponders which receive input optical signals from the other client equipments. The signal output from the MUX 17 is the combined multichannel signal and it is split into two paths via a 1×2 fibre coupler 19, each split portion of the signal being transmitted on an own optical fibre of two optical fibres used in the considered direction between the nodes. The first optical fibre 21 is called the working optical fibre and carries the light signal which is normally used. The second optical fibre 23 is called the protecting optical fibre and normally the information possibly carried thereon is not used. The light signals carried by the two parallel optical fibres 21, 23 are combined into one light signal by a 2×1 fibre coupler 25 which thus has its two inputs connected to the fibres 21, 23. The combined signal output from the coupler 25 is provided to a WDM demultiplexer (DEMUX) 27 which splits the combined signal into the different wavelength channels corresponding the wavelength specific input signals to provide a different single channel on each of the outputs of the demultiplexer. Each output of the demultiplexer 27 is connected to an individual receive end transponder (RET) 29, which receives the wavelength specific signal and forwards it to a client receiver 31.
Normally the optical signals arrive to the RETs 29 via an active optical preamplifier 33 connected to the output end of the working fibre 21 and to an input of the combining coupler 25. The signals are also propagating in the protection path all the way to a preamplifier 35 connected to the output end of the protecting fibre 23 and to the other input of the combining coupler 25. This preamplifier 35 is normally turned off, thus blocking the signals on the protecting fibre 23 which thus do not reach the combining coupler 25 and the demultiplexer 27. This is necessary since otherwise a receiver 31 would have the same signals coming through both the working and protecting fibre paths 21, 23 with different delays, which would cause interference cross talk and bit-errors.
In the case of the working fibre 21 being broken, the light power input to the working preamplifier 33 connected to this fibre will be lost. This causes, by suitable control lines, not shown, the working preamplifier 33 to be turned off and the preamplifier 35 connected to the protecting fibre 23 to be turned on in order to restore the traffic which will then be transmitted on the protecting fibre 23.
The client systems 13, 31 connected via a WDM system are often different types. Also the quality of services required from these client systems might vary. One example of a little more complicated system is illustrated by the diagram of FIG. 3. In FIG. 3 is shown some kind of IP equipments 13.1, 31.1 which are connected as the client equipments 13, 31 in FIG. 2 and which communicate with each other on a wavelength channel using wavelength specific signals. A 1+1 MSP SDH system 13.S, 31.S uses one channel of the WDM system as one of the fibres of the working pair of the SDH system and has a separate fibre 37 constituting that fibre of the protecting fibre pair for the SDH-system which carries light signals in the same direction. Other client systems, not shown, can communicate on the remaining wavelength channels. In this example the IP system comprising the IP-equipments 13.1, 31.1 has no protection except for that provided by the 1+1 OMSP in the WDM system. The SDH system 13.S, 31.S has duplicated client transmitters and receivers connected by an extra fibre 37 outside the WDM fibres.
A problem associated with the SDH system in this configuration in the case of the working WDM fibre 21 being cut is that the behaviour of the SDH system then is unpredictable. When the working preamplifier 33 in the WDM system looses its input power (due to the fibre cut), it is turned off. The traffic into the SDH equipment at the receiver end is consequently lost and a switch over to the special SDH protection fibre 37 is triggered, provided that the detection is fast enough, but is not immediately carried out. In parallel, the OMSP on the WDM system is carried out and the protection pre-amplifier 35 is turned on. When the WDM signal comes back the RETs 29 restarts and the signal into the client systems 31.1 (31.2 . . . 31.S) is restored. For the IP equipment 13.1, 31.1 this is the desired behaviour, but the SDH system 13S, 31 .S typically has a relatively slow protection procedure and has probably not switched over to the SDH protection fibre 37 yet. The traffic will then come back for a moment and then the switch over to the SDH protection fibre 37 might or might not take place.
Differently termed, if the trigger for the SDH protection is faster than the WDM restoration, then the SDH system will switch over to the standby transmitters and receivers on the SDH protection fibre 37, and if the WDM restoration is faster than the trigger for the SDH protection then the SDH traffic will remain on the WDM system, now on the WDM protection fibre 23.
From an operator's planning point of view this unknown outcome of a protection event is highly undesirable.